Experts are generally, but not always necessary in legal malpractice cases.  The test is whether a fact-finder can rely upon its own knowledge.  Here is an interesting case, Frances Northrop, respondent, v Eric Ole Thorsen, appellant. (Index No. 5684/04) ,2007-00973 ,
SUPREME COURT OF NEW YORK, APPELLATE DIVISION, SECOND DEPARTMENT ,
2007 NY Slip Op 10124; 2007 N.Y. App. Div. LEXIS 12903 , decided 12/18/07.

"In an action to recover damages for legal malpractice, a plaintiff must demonstrate that the attorney failed to exercise the ordinary reasonable skill and knowledge commonly possessed by a member of the legal profession and that the attorney’s breach of this duty proximately caused plaintiff to sustain actual and ascertainable damages. To establish causation, a plaintiff must show that he or she would have prevailed in the underlying action or would not have incurred any damages, but for the lawyer’s negligence. Expert testimony is normally needed to establish that the attorney failed to exercise the ordinary reasonable skill and knowledge commonly possessed by a member of the legal profession, unless the ordinary experience of the fact-finder provides sufficient basis for judging the adequacy of the professional service, or the attorney’s conduct falls below any standard of due care."

The case is interesting for two other points:  plaintiff won even after being precluded from using expert testimony; and plaintiff avoided a "mitigatino of damages" defense. "In support of his affirmative defense that the plaintiff failed to mitigate her damages, the defendant contends that the plaintiff herself could have avoided termination of her workers’ compensation benefits by making an application for nunc pro tunc judicial approval of the settlement. HN3The defendant, however, "may not shift to the client the legal responsibility [he] was specifically hired to undertake because of [his] superior knowledge"